1. Field of the Invention
The present invention relates to a retardation film, a method for producing the same and a liquid crystal display device. More particularly, the present invention relates to a retardation film having a structure where the retardation differs depending on the location and a method for producing the same. Furthermore, it relates to a liquid crystal display device using the retardation film having the above-mentioned structure or the substrate having a structure where the retardation differs depending on the location.
2. Description of the Related Art
Conventionally, a liquid crystal display device having a structure where optically isotropic polymers and optically anisotropic liquid crystal material are interposed between a pair of substrates has been used in a flat panel display apparatus for personal computers or the like, a liquid crystal TV apparatus, a portable display apparatus, etc. A retardation film according to the present invention can be used in these display apparatuses.
Conventionally, a retardation film is produced by drawing a polymer film in one direction so that polymer main chains are aligned in one direction. This creates anisotropy with respect to a refractive index in the polymer film so that a phase difference is produced between ordinary light and extraordinary light when light is transmitted through the retardation film.
Also, a method for controlling the tilt angles of the liquid crystal molecules using organic molecules is conventionally available. In Applied Physics No. 62, October (1993), a method is reported where a photoisomerization reaction of photochromic molecules is used.
The principle of this method is as follows. When linearly polarized light is irradiated onto certain photochromic molecules bonded to the substrate surface, structural isomerization from trans-form to cis-form occurs in the photochromic molecules. If a liquid crystal material is disposed on the substrate surface having the photochromic molecules bonded thereto in which such structural isomerization occurs, the tilt angle of the liquid crystal molecules with respect to the substrate can be varied in accordance with the structural change of the photochromic molecules.
Here, methods for forming an organic molecular layer having a desired pattern include, for example, a method where a polymerizable resin is applied onto the substrate by screen printing; and a method where a polymerizable resin which can be patterned such as a resist is applied onto the substrate and then, using a mask, the substrate is exposed to light so as to polymerize the resin.
As a technology for improving the mechanical strength of a panel in a liquid crystal display device, a structure has been proposed where stalls made of polymer, or the like, are disposed as reinforcing members against external mechanical force on the periphery of the portion in which a plurality of pixels are arranged.
For example, Japanese Laid-Open Patent Publication No. 56-99384 discloses a liquid crystal display device having a structure where polymer column members made of a resist are disposed in the portions other than the portions where the pixels are arranged, so as to be used as spacers.
Moreover, Japanese Laid-Open Patent Publication No. 59-201021 discloses a liquid crystal display device, which is similar to the one described in the above-mentioned publication, having a structure where wall members made of a photosensitive resin or the like are disposed in a stripe shape in the portions other than the portion where the pixels are arranged, so as to be used as spacers.
Furthermore, Japanese Laid-Open Patent Publication No. 6-301015, which corresponds to U.S. Pat. No. 5,473,450, discloses a liquid crystal display device having a reinforcing member formed by the phase separation of the mixture of the photopolymerizable material and the liquid crystal material. The mixture conducts the phase separation by light irradiation, so that the polymer forms the walls (reinforcing members) each of which encircles the pixel and the liquid crystal material is positioned inside the region encircled by the wall.
However, the following problems are present in the conventional method for producing a retardation film.
Since, in a conventional method for producing a retardation film by drawing, a polymer film is uniformly drawn in one direction so as to create anisotropy of a refractive index in the polymer film, it is considerably difficult to appropriately vary the retardation within a single sheet of retardation film depending on the location.
In a method where the tilt angles of the polymerized liquid crystal molecules are controlled by photochromic molecules formed on the substrate surface; the structural isomerization of photochromic molecules must be inclined the desirable portion of the substrate so that the tilt angles of the polymerized liquid crystal molecules are changed depending on the location within the retardation films. Therefore, the irradiation of linearly polarized light must have an irradiation intensity distribution on the plane of the substrate for producing the retardation film. However, it is considerably difficult to pattern the polarizing plate so as to create such intensity distribution.
Moreover, since the wavelength of light which induces polymerization of the polymerizable liquid crystal molecules and the wavelength of light which induces structural isomerization of the photochromic molecules are either the same or very close to each other, it is difficult to distinctly perform these two kinds of reactions. This makes it practically impossible to have the polymerization while the tilt angles of the polymerized liquid crystal molecules are held different depending on the location.
In a liquid crystal display device which has reinforcing members such as polymer columns or the like between the substrates constituting the liquid crystal panel, if a resist or a photopolymerizable material is used for forming the reinforcing members, the formed column or wall cannot have the same optical characteristics as the liquid crystal material. Therefore, when light is incident on the liquid crystal display device, the tone differs for light transmitted through the liquid crystal region and light transmitted through the polymer region. This makes it considerably difficult to control the tone for the entire expanse of the liquid crystal display device.
This phenomenon occurs because the polymer region is optically isotropic so that the tone created by the polarizing plates or the retardation films disposed on both sides of the liquid crystal panel appears as the tone of the polymer region without any change. This makes it difficult to control the tone for the panel. Moreover, such a coloring effect due to the polarizing plates or the like darkens the entire panel.